The invention relates generally to steam turbines. More particularly, the invention relates to mixing at least a portion of a flow of hotter steam, perhaps leaked from a first turbine, with cooler steam from a second turbine, and introducing the mixture into the second turbine.
Current turbo-machines such as steam turbines often employ multiple casings having turbines coupled to a common rotating shaft operable at different temperatures and pressures. For example, a steam turbine may include a high pressure (HP) casing, an intermediate pressure (IP) casing and a low pressure (LP) casing. Each casing may include a turbine having a plurality of stages therein with each stage including a row of blades that are coupled to the rotating shaft. Pressurized steam forces rotation of the blades as the steam presses against them and passes to the next stage. To contain the steam within a respective casing, each casing includes a series of non-contacting gland seals, referred to as an end packing. Each end packing includes a number of non-contacting seals such as leaf seals, brush seals, labyrinth seals, etc., that partially seal against the rotating shaft of the steam turbine. Due to the finite clearance in these seals there is an inevitable leakage of steam. The rate of leakage is dependent upon the seal geometry, clearance, and temperature difference between the steam inside the casing and the air outside the casing.
In steam turbines having an HP casing, a portion of steam that leaks through the HP end packing in excess of what is required by the steam seal regulator is dumped to a condenser, which results in lost potential to produce rotating shaft work. In theory, the steam could be admitted to the LP casing's turbine to generate useful work. However, in practice, the steam is too hot to admit it directly to the LP turbine because the material limit of LP turbine is exceeded, which makes it incapable of handling the temperature of the HP section steam.